Arrays of light emitting diodes are utilized for a wide variety of applications, including for ambient lighting and displays. For driving an array of LEDs, electronic circuits typically employ a power converter or LED driver to transform power from an AC or DC power source and provide a DC power source to the LEDs. When multiple LEDs are utilized, LED arrays may be divided into groups or channels of LEDs, with a group of LEDs connected in series typically referred to as a “string” or channel of LEDs.
Multichannel power converters are known, for example Subramanian Muthu, Frank J. P. Schuurmans, and Michael D. Pashly, “Red, Blue, and Green LED for White Light Illumination,” IEEE Journal on Selected Topics in Quantum Electronics, Vol. 8, No. 2, March/April 2002, pp. 333-338. Such prior art multistring LED drivers may utilize redundant power conversion modules, with a separate power module used for each LED string and typically comprising a driver, a transformer, a sensor, a controller, etc., for example. A similar approach is suggested in Chang et al., U.S. Pat. No. 6,369,525, entitled “White Light-Emitting-Diode Lamp Driver Based on Multiple Output Converter with Output Current Mode Control,” which utilizes multiple redundant power conversion modules, with each power conversion module configured to provide power for a corresponding LED string. Providing redundant elements such as a redundant power module for each channel may increase the number of components and may increase the size and weight of the power converter. Such utilization of relatively many components may also increase costs, such as component costs and manufacturing costs, or reduce reliability. For prior art power converters utilizing redundant power modules, a fault in a power module, such as if one or more components in the power module fail, may result in the power module no longer providing power or providing power at a reduced level and may cause a corresponding channel of LEDs to lose power.
Another prior art method (Supertex data sheets LV 9120/9123 and Application Note AN-H13) arranges LED strings in series and utilizes a power converter to provide power to the series arrangement of LED strings. In such an arrangement, the voltage level across the series of strings may be substantially equal to the sum of each voltage level across each of the multiple strings, resulting in an accumulated, total voltage level across multiple strings that may reach significantly high levels. FIG. 1 is a voltage map illustrating such voltage levels at the output of a prior art power converter and across a plurality of LED strings, for an example configuration in which the power converter drives four LED strings coupled in series. The vertical axis represents voltage “V.” Points along the horizontal axis represent corresponding points in the series configuration of LED strings. The first voltage level 20 for the “POWER CONVERTER OUTPUT,” marks the voltage rise across the output of the prior art power converter from substantially zero volts at the negative output terminal of the power converter to a total voltage VT at the positive output terminal of the power converter. The second voltage level 21 for an LED “FIRST STRING” illustrates the voltage drop across the first string of LEDs, the third voltage level 22 for an LED “SECOND STRING” illustrates the voltage drop across the second string of LEDs, and so on. As illustrated, the voltage level drops substantially to zero (24) across the fourth string. If the voltage across each string is 50V, for example, the total voltage level VT across the four strings or across the prior art power converter output is substantially equal to the sum of the voltage levels across each string, or 200V. Such relatively high voltage levels may make such a series arrangement unsuitable for some applications, such as where people may possibly come in contact with power provided to LED arrays. Operating at relatively high voltage levels may also incur additional costs for an apparatus, such as costs for components adapted to operate with such high voltage levels and for additional insulation and other safety equipment, such as to protect people and property. This prior art approach of providing power to a series of LED strings also does not provide a means for a controller to independently control the brightness of each string or to independently turn individual strings on or off.
Other prior art power converters with multiple power modules for multiple LED strings typically couple each load (e.g., channel or string of LEDs) to one of a plurality of power modules in a parallel configuration, i.e., a first terminal of the load is coupled to a first terminal of the power module and a second terminal of the load is coupled to a second terminal of the same power module. With such an arrangement, if one or more components in the power module fail, the load may lose power. Also, such an arrangement, in which each power module is coupled in parallel to a load, typically utilizes redundant circuitry, such as multiple sensors and multiple controllers, to provide a desired current level to multiple loads.
Accordingly, a need remains for a multichannel power converter that provides power to a plurality of LEDs, such as multiple strings or channels of LEDs, at comparatively low overall voltage levels, and that provides an overall reduction in size, weight, and cost of the LED driver, such as by sharing components across channels. Such a converter may further provide selected or predetermined power levels to the LEDs and may also compensate for variations in circuit parameters such as manufacturing tolerances, input voltage, temperature, etc. The power converter should be fault tolerant. For example, in the event that one or more power modules or channels fail, the power converter should continue to provide power to operational channels. Also, it would be desirable to provide a power converter adapted for providing independently selected power levels for each LED channel and for independently turning LED channels on or off.